U.S. patent application number 10/364624 was filed with the patent office on 2004-08-12 for multi-waveguide layer h-tree distribution device.
Invention is credited to Young, Ian, Zheng, Jun-Fei.
Application Number | 20040156591 10/364624 |
Document ID | / |
Family ID | 32824469 |
Filed Date | 2004-08-12 |
United States Patent
Application |
20040156591 |
Kind Code |
A1 |
Zheng, Jun-Fei ; et
al. |
August 12, 2004 |
Multi-waveguide layer H-tree distribution device
Abstract
An optical network is formed of multiple H-tree distribution
devices, separated into different waveguide layers. The optical
network receives an input optical signal, such as an optical clock
signal, and makes duplicate copies of that input signal. The
duplicate copies are routed through the connected H-tree
distribution devices, which are arranged to produce identical,
synchronized copies of the clock signal. The network can take the
form of a 1.times.2.sup.N device, where 2.sup.N represents the
number of these output signals. The H-tree distribution devices
forming the network are of varying size and may be formed in
different waveguide layers with different index of refraction
differentials between the H-tree devices and surrounding claddings.
In some forms, the optical network is integrated with
optical-to-electrical converters, i.e., photodetectors, which take
the optical output signals and convert them to synchronized
electrical signals that may be communicated to digital
circuits.
Inventors: |
Zheng, Jun-Fei; (Palo Alto,
CA) ; Young, Ian; (Portland, OR) |
Correspondence
Address: |
MARSHALL, GERSTEIN & BORUN LLP
6300 SEARS TOWER
233 S. WACKER DRIVE
CHICAGO
IL
60606
US
|
Family ID: |
32824469 |
Appl. No.: |
10/364624 |
Filed: |
February 10, 2003 |
Current U.S.
Class: |
385/45 ;
385/14 |
Current CPC
Class: |
G02B 6/125 20130101;
G02B 2006/1215 20130101; G02B 6/43 20130101 |
Class at
Publication: |
385/045 ;
385/014 |
International
Class: |
G02B 006/26 |
Claims
What we claim is:
1. An optical network device for distributing an input signal, the
optical network device comprising: an input waveguide for providing
the input signal; a first H-tree distribution device coupled to the
input waveguide and positioned at least partially at a first depth
of the optical network devices, the first H-tree distribution
device spanning a first area; and a second H-tree distribution
device coupled to the first H-tree distribution device and
positioned at least partially at a second depth of the optical
network device, the second H-tree distribution device spanning a
second area, wherein the second area is smaller than the first area
and wherein the first depth is different than the second depth.
2. The optical network device of claim 1, wherein the first H-tree
distribution device and the second H-tree distribution device are
formed in a single optical substrate.
3. The optical network device of claim 1, wherein the first H-tree
distribution device is formed in a first waveguide layer and
wherein the second H-tree distribution device is formed in a second
waveguide layer.
4. The optical network device of claim 3, wherein the first
waveguide layer is adjacent the second waveguide layer.
5. The optical network device of claim 3, wherein the first
waveguide layer has a first cladding region and wherein the first
H-tree distribution devices has a first index of refraction
differential with the first cladding region, and wherein the second
waveguide layer has a second cladding region and wherein the second
H-tree distribution device has a second index of refraction
differential with the second cladding region.
6. The optical network device of claim 5, wherein the first index
of refraction differential is different than the second index of
refraction differential.
7. The optical network device of claim 5, wherein the first index
of refraction differential is smaller than the second index of
refraction differential.
8. The optical network device of claim 5, wherein the first
cladding region and the second cladding region are formed of the
same material.
9. The optical network device of claim 1, wherein the second H-tree
distribution device is coupled to the first H-tree distribution
device through direct coupling.
10. The optical network device of claim 1, wherein the second
H-tree distribution device is coupled to the first H-tree
distribution device through evanescent coupling.
11. The optical network device of claim 1, further comprising a
coupler coupling the second H-tree distribution device and the
first H-tree distribution device.
12. The optical network device of claim 1, further comprising a
third H-tree distribution device coupled to the second H-tree
distribution device and positioned at least partially at a third
depth of the optical network device that is different from the
first depth and the second depth, the third H-tree distribution
device spanning a third area that is smaller than the second area
or the first area.
13. The optical network device of claim 1, wherein the first H-tree
distribution device is formed from a plurality of first Y-branches
each having a first radius of curvature and wherein the second
H-tree distribution device is formed from a plurality of second
Y-branches each having a second radius of curvature.
14. The optical network device of claim 13, wherein the first
radius of curvature is larger than the second radius of
curvature.
15. The optical network device of claim 13, further comprising a
third H-tree distribution device formed from a plurality of third
Y-branches each having a third radius of curvature, wherein the
third radius of curvature is smaller than the second radius of
curvature or the first radius of curvature.
16. The optical network device of claim 1, further comprising at
least one photodetector coupled to the second H-tree distribution
device and adapted to convert an optical signal propagating in the
second H-tree distribution device to an electrical signal.
17. The optical network device of claim 1, wherein the first H-tree
distribution device comprises a plurality of first Y-branches each
having a first width, and wherein the second H-tree distribution
device comprises a plurality of second Y-branches each having a
second width different than the first width.
18. The optical network device of claim 1, wherein the first H-tree
distribution device comprises a plurality of first Y-branches each
having a first thickness, and wherein the second H-tree
distribution device comprises a plurality of second Y-branches each
having a second thickness different than the first thickness.
19. A multi-layer optical device comprising: a first waveguide
layer having a cladding region and a first H-tree distribution
device; a second waveguide layer having a cladding region and a
second H-tree distribution device, wherein the second waveguide
layer is disposed adjacent to the first waveguide layer to couple
the first H-tree distribution device to the second H-tree
distribution device; and wherein the first H-tree distribution
device has a first index of refraction differential with the
cladding region of the first waveguide layer, and the second H-tree
distribution device has a second index of refraction differential
with the cladding region of the second waveguide layer, the second
index of refraction differential is different than the first index
of refraction differential.
20. The multi-layer optical device of claim 19, wherein the first
index of refraction differential is smaller than the second index
of refraction differential.
21. The multi-layer optical device of claim 19, wherein the second
H-tree distribution device is coupled to the first H-tree
distribution device through direct coupling.
22. The multi-layer optical device of claim 19, wherein the second
H-tree distribution device is coupled to the first H-tree
distribution device through evanescent coupling.
23. The multi-layer optical device of claim 19, further comprising
a coupler coupling the first waveguide layer and the second
waveguide layer.
24. The multi-layer optical device of claim 19, wherein the first
H-tree distribution device spans a first area, and wherein the
second H-tree distribution device spans a second area that is
smaller than the first area.
25. The multi-layer optical device of claim 24, wherein the first
H-tree distribution device is formed from a plurality of first
Y-branches each having a first radius of curvature and wherein the
second H-tree distribution device is formed from a plurality of
second Y-branches each having a second radius of curvature.
26. The multi-layer optical device of claim 25, wherein the first
radius of curvature is larger than the second radius of
curvature.
27. A method of forming an optical distribution device for
receiving an input signal, the method comprising: forming an input
waveguide for propagating the input signal; coupling a first H-tree
distribution device in a first waveguide layer to the input
waveguide, the H-tree distribution device spanning a first area;
and coupling a second H-tree distribution device in a second
waveguide layer to the first H-tree distribution device, the second
H-tree distribution device spanning a second area different than
the first area.
28. The method of claim 27, wherein the first waveguide layer has a
first cladding region and wherein the first H-tree distribution
device has a first index of refraction differential with the first
cladding region; and wherein the second waveguide layer has a
second cladding region and wherein the second H-tree distribution
device has a second index of refraction differential with the
second cladding region that is greater than the first index of
refraction differential.
29. The method of claim 28, wherein the first cladding region and
the second cladding region are formed of the same material.
30. The method of claim 27, further comprising coupling the second
H-tree distribution device to the first H-tree distribution device
through direct coupling.
31. The method of claim 30, further comprising disposing a coupler
between the second H-tree distribution device and the first H-tree
distribution device.
32. The method of claim 27, further comprising coupling the second
H-tree distribution device to the first H-tree distribution device
through evanescent coupling.
33. The method of claim 27, further comprising evanescently
coupling a photodetector to the second H-tree distribution
device.
34. The method of claim 27, further comprising directly coupling a
photodetector to the second H-tree distribution device.
35. The method of claim 27, wherein coupling the first H-tree
distribution device comprises forming a first plurality of
Y-branches within the first waveguide layer, at least one of the
plurality of first Y-branches having a first radius of curvature,
and wherein coupling the second H-tree distribution device
comprises forming a plurality of second Y-branches within the
second waveguide layer, at least one of the plurality of second
Y-branches having a second radius of curvature different than the
first radius of curvature.
36. The method of claim 27, further comprising: forming a first
plurality of Y-branches within the first waveguide layer, each of
the first plurality of Y-branches having a first width; and forming
a second plurality of Y-branches within the second waveguide layer,
each of the second plurality of Y-branches having a second width
different than the first width.
37. The method of claim 27, further comprising: forming a first
plurality of Y-branches within the first waveguide layer, each of
the first plurality of Y-branches having a first thickness; and
forming a second plurality of Y-branches within the second
waveguide layer, each of the second plurality of Y-branches having
a second thickness different than the first thickness.
38. A method of distributing an input signal comprising:
propagating the input signal on an input waveguide; coupling the
input signal into a primary H-tree distribution device coupled to
the input waveguide, the primary H-tree distribution device formed
in a first waveguide layer and spanning a first area; and in
response to coupling the input signal into the primary H-tree
distribution device, forming a first plurality of output signals;
coupling at least one of the plurality of output signals to at
least one secondary H-tree distribution device formed in a second
waveguide layer and spanning a second area different than the first
area.
39. The method of claim 38, further comprising: forming the second
waveguide layer adjacent the first waveguide layer to couple the
secondary H-tree distribution device to the primary H-tree
distribution device.
40. The method of claim 38, further comprising: propagating at
least a portion of the input signal through a plurality of first
Y-branches, each having a first radius of curvature; and
propagating at least a portion of the at least one the plurality of
output signals through a plurality of second Y-branches, each
having a second radius of curvature different than the first radius
of curvature.
41. The method of claim 38, wherein the first waveguide layer has a
first cladding region and wherein the primary H-tree distribution
device has a first index of refraction differential with the first
cladding region; and wherein the second waveguide layer has a
second cladding region and wherein the secondary H-tree
distribution device has a second index of refraction differential
with the second cladding region that is greater than the first
index of refraction differential.
Description
FIELD OF THE DISCLOSURE
[0001] This patent generally relates to signal propagation and more
specifically to H-tree distribution of a signal.
BACKGROUND OF THE PRIOR ART
[0002] For digital systems, accurate timing is crucial to data
transmission. Clocking signals therefore are crucial to digital
systems, because clocking signals set the timing for the components
in the systems. A computer motherboard, for example, might have a
single master clock signal that is transmitted to and synchronized
with integrated circuit boards, chipsets, peripherals, or other
components connected to the motherboard. All system components may
be synchronized using this master clock signal.
[0003] Various techniques exist for generating and distributing
clock signals within a digital system. For example, a primary clock
signal might be generated by a ring oscillator or a separate clock
chip (e.g., a crystal oscillator). The clock signal may then be
routed from the generator to each of the devices connected to the
clock. These techniques use electrical clock signals, i.e., clock
signals traveling along metallic or semiconductor conduits.
Unfortunately, clock signals in the electrical domain present
numerous design limitations.
[0004] Ideally, clock signals would have a well defined duration,
consistent shape, and zero propagation path dependence. In reality,
clock signals have variable rise and fall times, noticeable jitter,
and noticeable path-dependent skew, a particular problem that
arises from timing differences and waveform variation between clock
signals. There is also a sizeable power drain associated with
electrical clock signals.
[0005] Typically, clock signals are distributed throughout a system
via a distribution network. In theory, the network would make
duplicate copies of a clock signal and provide identical paths for
each duplicate copy, so that any device connected to the network
would receive a synchronized clock signal. In reality, however,
skew and jitter problems abound, primarily due to electrical load
differences among the various paths and parasitic effects within
the network.
[0006] Recently, some have proposed moving away from a purely
electrical domain digital clocking system to an optical domain
clocking system. Using optical signals, i.e., light pulses,
presents some obvious theoretical advantages. Optical signals are
not susceptible to load variations or parasitic effects, because
they travel through waveguides and not conducting metallic wires.
Also, optical signals may propagate at much faster clock rates.
Electrical clock signals have a theoretical limit of about 25 GHz
for signal transmission of about 5 mm, while optical clock signals
may extend into the THz range and travel much longer distances,
thus allowing for a digital system with orders of magnitude faster
performance capabilities.
[0007] In the optical clock networks proposed for clock signal
distribution, a network distributor generates or receives a clock
signal, and that signal is then split into multiple signals by a
Y-branch splitter, multimode interferometer or similar device. Each
copy of the clock signal is then provided to one output waveguide,
where all the output waveguides are of equal length to keep the
copies of the clock signal in phase.
[0008] While optical networks do not have the impedance load
variation and parasitic problems of electrical domain networks,
they have their share of shortcomings. One of the main problems
affecting optical networks is modal confinement. For an optical
signal to propagate and not lose its waveform or intensity, the
signal's mode must be confined to the propagating waveguide.
Further still, its mode profile must stay constant over the
propagation length of that waveguide. This means that the
waveguides must have a higher index of refraction differential with
respect to their surrounding cladding layers. This also means that
only waveguides of certain bending radii (typically quite large)
are used to avoid bending losses. Unfortunately, large bending
radii result in large devices and, as such, limit device
scalability. The problem is multiplied with network complexity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is an illustration of a device having a clock
distribution device and a digital microchip.
[0010] FIG. 2 is an illustration of an optical distribution device
with an H-tree distribution network that may be used as the clock
distribution device of FIG. 1.
[0011] FIG. 3 is an exploded illustration of a portion of the
H-tree distribution network of FIG. 2, showing H-tree distribution
devices of different size.
[0012] FIG. 4 is a partial illustration of two waveguide layers
each containing at least one H-tree distribution device.
[0013] FIG. 5 is an illustration of a series of interconnected
H-tree distribution devices of differing size.
[0014] FIG. 6 is a side illustration of the structure of FIG. 5,
taken along line AA, along with additional structure, showing
evanescent coupling between H-tree distribution devices.
[0015] FIG. 7 is an illustration of an example coupling between an
output waveguide of an H-tree distribution device and a
photodetector.
[0016] FIG. 8 is an illustration of another example coupling
between an output waveguide of an H-tree distribution device and a
photodetector.
[0017] FIG. 9 is a side illustration, similar to that of FIG. 6, of
another coupling between H-tree distribution devices.
[0018] FIG. 10 illustrates a system that includes an optical
network routing an optical signal to a plurality of subsystems.
DETAILED DESCRIPTION
[0019] Various optical devices are described. Although the
descriptions are provided in the context of propagating an optical
clock signal, it will be understood by persons of ordinary skill in
the art that the examples are not limited to the transmission of
optical clock signals. The devices described may be used to
distribute any information carrying optical signal. Furthermore,
while the techniques described are provided in the context of
distributing an input optical signal into a plurality of output
signals, the techniques may be used on any number of optical
devices to provide increased scalability and performance.
[0020] FIG. 1 illustrates an example device 100 that includes an
integrated chip module 102 mounted on a substrate 104 that may be a
printed circuit board, for example. The integrated chip module 102
includes a mounting substrate 106 which may be a DIP package or
Ball Grid Array (BGA), for example. The integrated circuit module
102 also includes a clock distribution device 108 and a digital
microchip 110 mounted to the clock distribution device 108. In the
illustrated configuration, the clock distribution device 108 is
mounted directly to the mounting substrate 106.
[0021] The digital chip 110 may be any known digital chip including
a microprocessor, an application specific integrated chip (ASIC),
chipset, or the like. Example digital chips include those from the
Intel Corporation family of microprocessors, of which the Intel
Pentium.RTM. microprocessor is an example. The digital chip 110 may
represent a device having multiple subsystems that are each capable
of receiving a separate clock signal.
[0022] The clock distribution device 108 is an optical network
capable of creating and distributing multiple clock signals in an
optical domain. Further, the distribution device 108 is an
integrated device with optical-to-electrical (O/E) converters that
convert optical domain clock signals into electrical domain clock
signals, which may then be provided to the digital chip 110 via a
BGA 112, in the illustrated example. The O/E converter is also
termed a photodetector herein, of which a photodiode is an example.
Also, the distribution device 108 may include output waveguides
without O/E conversion to provide optical clock signals.
[0023] In operation, the distribution device 108 is fed with a
master clock signal via an input waveguide 114. The clock signal
may be from an external waveguide such as an optical fiber coupled
to a clock generating circuit, not shown. The master clock signal
is routed throughout the distribution device 108, which creates as
many copies of the master clock signal as desired. By way of
example only, the distribution device 108 may be a
1.times.2.times.2 (1 to 4), 1.times.2.times.2.times.2.times.2 (1 to
16), 1.times.2.times.2.times.2.times.2.times.2.times.2 (1 to 64),
or 1.times.2.sup.N distribution network, taking in one input clock
signal and, in the latter example, generating 2.sup.N identical
versions of that clock signal.
[0024] The distribution device 108, being an optical distribution
network, will propagate clock signals of a much higher frequency
than is achievable with traditional electrical distribution
devices. Also, the distribution device 108 produces clock signals
with reduced skew and reduced jitter, i.e., with less variation
between the clock signals as compared to electrical distribution
devices due to equal propagation length management by the optical
network within the device 108.
[0025] FIG. 2 illustrates a top view of an example distribution
device 200 that may be used as the distribution device 108. The
distribution device 200 is an optical device with two input optical
waveguides 202 and 204. The input waveguide 202 is connected to an
H-tree distribution device 206 formed in a first section 208 and
formed of three interconnected Y-branches 210, 212, and 214. Output
branches 216 and 218 of the Y-branch 210 couple as inputs to the
Y-branches 212 and 214, respectively. The Y-branch 212 has output
branches 220 and 222, and the Y-branch 214 has output branches 224
and 226. The three Y-branches 210, 212, and 214 form the H-tree
distribution device 208, which is a 1.times.2.times.2 distribution
device.
[0026] With the Y-branches 212, 214, and 216 each being 50/50
splitters and each being substantially the same in shape and
dimension, the output branches 220, 222, 224, and 226 will transmit
substantially identical copies of the input clock signal on the
waveguide 202. Photodetectors 228, 230, 232 and 234 are used for
O/E conversion of the signal on those output branches 220, 222,
224, and 226, respectively. The photodetectors 228, 230, 232 and
234 may be silicon photodiodes, for example.
[0027] The input waveguide 204 is connected to a first H-tree
distribution network 250 in a second section 251 of the device 200.
The network 250 has 32 output waveguides each connected to an O/E
converter 252 and each providing an identical copy of an input
signal received on the input waveguide 204. For readability, not
all of the illustrated O/E converters 252 in the network 250 are
numbered with a reference numeral. Nonetheless, the converters 252
are substantially identical in the illustrated example to ensure
that the clock signals produced by the network 250 are in phase
when converted to the electrical domain. The H-tree network 250 is
formed from a plurality of interconnected individual H-trees
distribution devices. As illustrated in FIG. 3, the waveguide 204
is an input waveguide for a first H-tree distribution device 254,
or primary H-tree distribution device. The H-tree distribution
device 254 includes four output waveguides 256, 258, 260, and 262,
each carrying a substantially identical output signal derived from
the input signal coupled from the input waveguide 204. The output
signals from the device 254 are attenuated copies of the input
signal but are still in phase with one another. The output
waveguide 258 is coupled to a secondary H-tree distribution device
264, which has two output waveguides 266 and 268 coupled to H-tree
distribution devices 270 and 272, respectively. As with the H-tree
distribution device 254, each H-tree distribution device 264, 270,
and 272 receives a signal, an output signal from a next larger
H-tree distribution device, and creates four substantially
identical output signals. In an example, the input signal may be an
optical clock signal, for example, one produced by a mode locked
laser source.
[0028] As depicted in the illustrations of FIGS. 2 and 3, the
network 250 is formed of individual H-tree distribution devices of
differing size. For example, the H-tree distribution device 254 is
a level 1 structure that spans a larger area, as shown, than the
H-tree distribution device 264, which is a level 2 structure.
Further, the H-tree distribution device 264 spans a larger area, as
shown, than the level 3H-tree distribution devices 270 and 272,
which, in the illustrated example, span an identical area. The
level indications used herein are for convenience purposes to
describe H-tree distribution devices of different size in area and
that exist within different waveguide layers, as will be described
in more detail below. The area spanned by a particular H-tree
distribution device, in plan view, includes at least the three
Y-branches forming that device including at least a portion of the
input and out waveguides for each of the three Y-branches.
[0029] In the illustrated example, the differences between
structures of different levels are not only differences in overall
area, but are also differences in Y-branch radius of curvature. The
H-tree distribution device 254, for example, is formed of three
Y-branches 274, 276, and 278 (see FIG. 3), each with branches of
identical radius of curvature. This radius of curvature is larger
than the radius of curvature for three Y-branches 280, 282, and 284
forming the H-tree distribution device 264. That is, the radius of
curvature on the level 1 structure is larger than the radius of
curvature on the level 2 structure(s). Similarly, the radius of
curvature on the Y-branches 280, 282, and 284 is larger than the
radius of curvature of the Y-branches (not labeled) forming the
level 3 structures 270 and 272.
[0030] The network 250 is symmetrical and provides substantially
identical input signals at the O/E converters 252. As a result,
although not depicted in FIG. 3, there is a second level 2 (or
secondary) H-tree distribution device, substantially identical to
assembly 264, coupled to each of the output waveguides 254, 256,
and 262. Further, each of these level 2 assemblies are connected to
four level 3 assemblies that are themselves identical to one
another, and for the illustrated example, would be identical to
assemblies 270 and 272. The assembly 264 is shown in FIG. 3 coupled
to only two such level 3 structures (270 and 272) for
simplification purposes only. FIG. 2 illustrates that the input
waveguide 204 may be coupled to the first H-tree distribution
network 250 and a second H-tree distribution network 286 that is
identical to the network 250 and, as such, not described further
herein.
[0031] In addition to indicating relative size, the level
indications on various structures depicted herein may also describe
the location of the structure relative to other structures. FIG. 4
is a partial illustration of a device 300 formed of two waveguide
layers 302 and 304 disposed adjacent one another. All of the
waveguide layers described herein may provide a cladding region and
a waveguide core region. The first waveguide layer 302 includes a
level 1 structure in the form of an H-tree distribution device 306
(partially shown) that may be formed in the layer 302 through
deposition, photolithographic, chemical etch, deposition again, and
planarization processing, for example. A first Y-branch 308, a
second Y-branch 310, and output waveguides 312 and 314 of the
H-tree distribution device 306 are shown, and all are formed in the
layer 302. This layer 302 may be deposited, formed, clamped, or
bonded on the layer 304, after a level 2H-tree distribution device
316 (partially shown) has been formed therein. FIG. 4 illustrates a
Y-branch 318 of the H-tree distribution device 316 having branching
waveguides 320 and 322. In line with the examples of FIGS. 2 and 3,
the H-tree distribution device 306 has a radius of curvature,
R.sub.1. As illustrated, the H-tree distribution device 316 has a
radius of curvature, R.sub.2, where R.sub.1>R.sub.2. That is, a
tighter radius of curvature is used in the level 2 structure 316.
As the illustration of FIG. 4 shows, the device 300 has the H-tree
distribution device 306 formed at a first depth of the device 300
and the H-tree distribution device 316 formed at a second depth of
the device 300, wherein the two depths are different The devices
306 and 316 may have portions that extend into or from different
depths. For example, an input waveguide coupled to the first
Y-branch 308 may extend from a different depth of the device 300 or
from a different waveguide layer, not shown.
[0032] The waveguide layers 302 and 304 may be formed of the same
cladding material, for example a SiO.sub.2 material as the cladding
layer. Alternatively, each layer 302 and 304 may be formed of a
different material. A SiO.sub.xN.sub.y material, with x between and
including 0 to 2 and y between and including 0 to 1.333, may be
used as the waveguide core material. These x and y values are
provided by example and apply to all SiO.sub.xN.sub.y materials
herein. Because the radii of curvature on the devices 306 and 316
are different, different SiO.sub.xN.sub.y materials may be used in
each of the layers 302 and 304. For example, the device 316, having
a tighter radius of curvature than device 306, may have a
SiO.sub.xN.sub.y material that produces a higher index of
refraction differential between the waveguides and surrounding
cladding, than would the SiO.sub.xN.sub.y material used with the
device 306, which has a larger radius of curvature. As used herein,
the term index of refraction differential, also referred to as the
index of refraction contrast, refers to the difference between the
index of refraction of a waveguide core region and a cladding
region surrounding the core region, at a given reference
wavelength.
[0033] Coupling of an optical clock signal for the output
waveguides 312 and 314 into level 2 structures is achieved by
evanescent coupling in the depicted illustration, i.e., by having
waveguides adjacent to one another so that energy may flow between
the two. Other coupling, such as direct or butt-coupling may also
be used. Specifically, the waveguide 312 is disposed above or
adjacent the input waveguide 324, thereby defining an overlapping
or coupling length, D. In the illustrated example, the length D is
chosen to ensure maximum coupling of energy. The waveguide 314
would similarly share a coupling length with a second level 2
structure (not shown) in the same manner. Furthermore, it will be
understood that the level 2 structure 316 may also include output
waveguides that are coupled to O/E converters or coupled to other
structures like a level 3 structure in a third waveguide layer. In
either case, the O/E converters or additional structure may
alternatively be formed in the layer 304. In fact, while FIG. 4
shows two different waveguide layers 302 and 304, the H-tree
distribution devices 306 and 316 may be formed at different depths
in a single optical substrate and doped or fabricated to have
different indices of refraction if desired.
[0034] FIG. 5 illustrates a portion of an H-tree distribution
network 400 formed of a level 1H-tree distribution device 402, a
level 2H-tree distribution device 404, and two level 3H-tree
distribution devices 406 and 408. In the illustrated example, each
level indication represents a structure of different waveguide
dimension, width (FIG. 5) and height (FIG. 6). For example, an
output waveguide 410 of the H-tree distribution device 402 may be
wider than an input waveguide 412 of the H-tree distribution device
404 coupled thereto. An overlap between the waveguides 410 and 412
is shown for explanation purposes.
[0035] FIG. 6 is an illustration of a side view of the device 400
taken along line AA of FIG. 5, with additional structure shown. The
H-tree distribution device 402 is formed in a first waveguide layer
450; the H-tree distribution device 404 is formed in a second,
smaller thickness waveguide layer 452; and the H-tree distribution
device 406 is formed in a third, even smaller thickness waveguide
layer 454. Further, the substrates 450, 452, and 454 are disposed
adjacent one another, as illustrated, thereby allowing energy form
one layer to couple into an adjacent layer. The waveguide layers
450, 452, and 454 need not be in this configuration. Instead, a
cladding or buffer region may extend between layers. In the
illustrated example, the H-tree distribution devices 402 and 404
share a coupling region 456. Similarly, the region 458 couples the
H-tree distribution devices 404 and 406. Evanescent coupling occurs
over the regions 456 and 458, in the illustrated embodiment.
Alternatively, coupling between distribution devices may be through
directional coupling, such as extending a coupling region between
devices that otherwise do not touch one another. FIG. 9, described
in more detail below, illustrates an example with a coupler
extending between adjacent distribution devices.
[0036] With structures of different size formed in different
waveguide layers, a network device can be formed of H-tree
distribution devices having different optical properties. In the
illustrated example of FIGS. 5 and 6, the H-tree distribution
device 402 is a level 1 structure with a level 1 index of
refraction that is smaller than a level 2 index of refraction for
the H-tree distribution device 404. Thus, if cladding regions 460
and 462 of the layers 450 and 452, respectively, have substantially
the same index of refraction, the index of refraction differential
(or contrast) between waveguide and cladding for the H-tree
distribution device 402 (i.e., .DELTA.n1) will be smaller than the
index of refraction differential (.DELTA.n2) for the H-tree
distribution device 404. Similarly, in the illustrated example,
.DELTA.n3 for the H-tree distribution device 406 in the layer 454
having cladding region 464 is larger than .DELTA.n2. The cladding
regions 460, 462 and 464 in the illustrated example may be made of
SiO.sub.2, for example, and the H-tree distribution devices 402,
404, and 406 may be made of SiO.sub.xN.sub.y, where x and y are
chosen to form the desired .DELTA.n ratios. Example values are
given above.
[0037] For optical distribution devices of increasingly smaller
sizes, the index of refraction differential may be increased to
reduce any bending losses on the propagating optical signal. By way
of example only, the radii of curvature of the Y-branches described
may range from 500 .mu.m to sub 10 .mu.m and the index of
refraction differential, An, may range from approximately 0.05 to
approximately 1, with a larger differential corresponding to
smaller radii of curvature. These ranges are merely examples,
however, and the radii of curvature and index of refraction
differential may be chosen in any manner to minimize losses and
increase scalability for any desired size of the optical
distribution structure.
[0038] To provide O/E conversion, a fourth waveguide layer 466 is
disposed adjacent the layer 454 and houses a photodetector 468
coupled to the H-tree distribution device 406. In this three level
configuration, the level 3H-tree distribution device 406 provides
the output waveguides for the assembly 400. The photodetector 468
may be a silicon-based photodiode, for example, and is shown
evanescently coupled to the H-tree distribution 406. The
photodetector 468 may represent a separate device coupled to the
H-tree distribution device 406. Other photodetection methods may
also be used.
[0039] FIG. 7 shows another example of evanescent coupling in which
an output waveguide 500 of an H-tree distribution device is
disposed adjacent a photodetector 502 extending into a waveguide
layer 504. The photodetector 502 converts an optical clock signal
coupled from the waveguide 500 into an electrical signal, which may
be communicated to circuitry through a conducting lead 506. FIG. 8
shows a similar structure to that of FIG. 7, but with the waveguide
500 direct or butt-coupled to a photodetector 508 formed at least
partially extending above a surface of a waveguide layer 510. An
anti-reflection coating may be used between the waveguide 500 and
the butt-coupled photodetector 508 to reduce reflection loss. A
conducting lead 512 extends from an opposite end of the layer 510
to couple an electrical clock signal to external circuitry. Other
techniques, such as angling adjacent faces of the waveguide 500 and
the photodetector 508, may also be used. In both FIGS. 7 and 8, the
waveguide 500 may be a stand alone structure or may be formed in a
layer (not shown) similar to that shown in FIG. 6.
[0040] FIGS. 7 and 8 show example techniques for integrating an
optical-to-electrical converter into a multilayer distribution
network by forming a layer with a photodetector adjacent the output
waveguides of an H-tree distribution network. For example, if the
smallest structure in a network (e.g., a level 3H-tree distribution
device in a three level structure) is formed in a bottom waveguide
layer, a photodiode substrate layer may be coupled to this bottom
waveguide layer. It will be understood by persons of ordinary skill
in the art that other techniques may be used for such coupling and
conversion. Furthermore, it will be understood that the
photodetectors 468, 502, and 508 may be silicon-based structures or
formed of other materials. Germanium-based materials, for example,
may be particularly useful for converting optical signals
propagating at approximately 1550 nm or 1310 nm, whereas
silicon-based photodetectors may be desired for wavelengths such as
approximately 850 nm or shorter wavelengths such as 400-650 nm.
[0041] FIG. 9 shows an optical device 600 with a generic coupling
structure, where a first H-tree distribution device 602 is coupled
to a coupler 604 that is also coupled to a second H-tree
distribution device 606. The coupler 604 may be a coupling region,
like a bulk region through which modal transformation between the
structures 602 and 606 occurs. The coupler 604 may be a tapered
waveguide or a prism or mirror that reflects energy from the
structure 602 into the structure 606. Other coupling devices,
whether based on modal transformation, like an interference-based
optical structure, reflection, or the like will be known to persons
of ordinary skill in the art. A similar coupler 608 couples the
H-tree distribution device 606 with the H-tree distribution device
610, which is, in turn coupled to a photodetector 612.
[0042] FIG. 10 illustrates a system 1000 that provides a clock
signal, or other optical signal, to devices within the system 1000
or devices connected thereto. By providing identical copies of the
clock signal, these devices may be synchronized together.
[0043] The system 1000 includes a clock signal generator 1002
coupled to an optical network 1040. The clock signal generator may
be a 10 GHz mode locked laser providing an optical clock signal,
such lasers are known and may have better than 100 fs jitter. An
optical fiber or waveguide may be used in coupling the generator
1002 to the network 1004. Other optical clock signal generators may
also be used. The generator 1002 may alternatively provide an
electrical clock signal that may be coupled to either the network
1004 directly or a separate device, where the electrical clock
signal is converted to an optical clock signal. For example, an
electrical clock signal may be used to modulate a laser to create
an optical clock signal. The clock signal generator 1002 and the
optical network 1004 may be integrated in a computer motherboard or
on a microchip, or they may be separate devices.
[0044] The network 1004 is coupled to a series of subsystems Sub1
1006, Sub2 1008, Sub3 1010., SubN-2 1012, SubN-1, 1014, and SubN
1016, providing an optical clock signal to each. The subsystems
1006-1016 may represent a circuit, microprocessor, chipset, memory,
I/O interface, or other device that typically receives a clock
signal in a processor system. The generator 1002 provides a clock
signal to the network 1004, which then creates identical copies of
the clock signal. The copies are synchronized with the clock signal
from the generator 1002 and are substantially identical in
intensity and are in-phase. The network 1004 provides at least one
copy to each of the subsystems 1006-1016. The network 1004 is also
connected to a separate subsystem 1018 that may receive a different
clock signal from that sent to the subsystems 1006-1016.
[0045] The network 1004 may be a network of individual H-tree
distribution devices like any of those described hereinabove.
[0046] Although certain apparatus constructed in accordance with
the teachings of the invention have been described herein, the
scope of coverage of this patent is not limited thereto. On the
contrary, this patent covers all embodiments of the teachings of
the invention fairly falling within the scope of the appended
claims either literally or under the doctrine of equivalence.
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